Arsect the Venomous

From Diablo Wiki

Arsect the Venomous is a SuperuniqueSpider that appears in the Caverns of Araneae near Wortham in Act I. The exact probability of appearance is not known. In one case when this was recorded, Arsect spawned 6 times in 40 attempts (15% chance).

Bonus Treasure Goblin

It will be guarding a fat chest and a cocooned victim who you can free after killing Arsect. It is a Treasure Goblin.